1. Field of the Invention
The present invention relates to a method of quenching metals.
2. Description of the Prior Art
A heat treating oil is called a quenching oil and is used to increase the mechanical strength of metal, but cannot provide a rust preventive ability and lubricity. Thus, various attempts have been made to provide such a rust preventive ability and lubricity, including a surface treatment in which a black coating layer is formed on the surface of metal while at the same time achieving the quenching of the metal. For this purpose, several heat treating oil compositions have been proposed as described in, for example, Japanese Patent Publication Nos. 20806/1972, 32288/1974, and 33484/1975. In fact, a heat treating oil composition comprising a base oil consisting of a mineral oil, and zinc alkyldithiophosphate is widely used.
This heat treating oil composition, however, creates various problems, for example:
(1) At a quenching step, particularly at a carburizing quenching step, the zinc compound decomposes upon heating and evaporates, depositing on the metal surface and preventing the formation of the desired black coating layer thereon;
(2) Since the reactivity of the zinc compound with metals is low, the adhesive properties and rust preventive ability of the coating layer is inferior, and furthermore, on a planished surface and a bonderized surface, the black coating layer is formed only unevenly;
(3) The coefficient of friction of the metal that has been quenched is high, and characteristics, such as anti-seizuring properties and fatigue life (anti-pitting life), are not sufficiently satisfactory, which is a serious problem particularly in the case of parts, such as constant velocity joints; and
(4) In the case of bolts which are tempered at high temperatures of from 500.degree. to 600.degree. C., the desired coating layer is not formed by a treatment starting at the tempering temperature, because the reactivity of the zinc compound is low.